The progressive desensitization which develops during closely repeated agonist applications or sustained agonist exposure is a common, but poorly understood consequence of almost all agonist- receptor interactions. The activation- desensitization sequence has been studied most extensively for the nicotinic AchR channel complex at the motor endplate of skeletal muscle or electric organs. In contrast, no comprehensive study of this important process has been completed for nicotinic AChR-channel complexes in vertebrate neurons. Therefore, the primary objective of the proposed study is to establish the kinetics of the activation- desensitization sequence occurring at nicotinic AChR-channel complexes in amphibian autonomic postganglionic neurons. These experiments will also test whether desensitization of the nicotinic receptor-channel complex occurs under physiological conditions and therefore is a mechanism by which the efficiency of ganglionic transmission can change during trains of repetitive preganglionic stimulation. The studies will be done using bullfrog sympathetic and parasympathetic ganglion cells. Individual ganglion cells in intact preparations and single enzyme dissociated cells will be utilized. Membrane currents will be measured in individual cells within intact ganglia using either a two microelectrode or a single electrode voltage clamp system. Whole cell patch clamp measurements of membrane currents will be obtained from enzymatically dissociated cells. The specific aims are to: 1. Establish the basic kinetic properties of desensitization of the nicotinic AChR in vertebrate ganglion cells. 2. Establish whether receptor channel activation kinetics determine desensitization kinetics in autonomic ganglion cells. 3. Establish whether AChR-channel desensitization is a physiological mechanism involved in integration within ganglia. The results of these experiments will provide important new information about the control of nicotinic-gated channels in vertebrate neurons.